1. Field of the Invention
The present invention relates to a high-frequency circuit for use in a low-noise high-frequency amplifier or the like of a satellite broadcasting low-noise block down-converter. The present invention, particularly, relates to a high-frequency circuit having bias lines that cross a microstrip line in a plan view.
2. Description of the Prior Art
A part of a low-noise high-frequency amplifier, used in an LNB (Low-noise Block Down-converter) that receives a signal from a broadcasting satellite or a communications satellite and outputs an intermediate-frequency signal after frequency conversion, is shown in FIG. 5 as an example of a conventional high-frequency circuit. A low-noise high-frequency amplifier 10 of the LNB includes an MIC (Microwave Integrated Circuit) comprising a substrate, microstrip lines 14L and 14R formed thereon, and elements built in the microstrip lines.
After received by an antenna (not illustrated), radio-frequency signals in 12-GHz band in the form of a left-handed polarized wave and a right-handed polarized wave are fed to the microstrip lines 14L and 14R respectively through input terminals 11L and 11R thereof.
The input signal in the form of a left-handed polarized wave fed through the input terminal 11L is outputted from an output terminal 12L after having been amplified by two amplifiers 13L1 and 13L2, both of which are built in the microstrip line 14L. The input signal in the form of a right-handed polarized wave fed through the input terminal 11R is outputted from an output terminal 12R after having been amplified by two amplifiers 13R1 and 13R2, both of which are built in the microstrip line 14R.
Each of the amplifiers 13L1, 13L2, 13R1, and 13R2 comprises a GaAsFET (Gallium Arsenide Field-Effect Transistor). Between the amplifiers 13L1 and 13L2 in the microstrip line 14L is formed a coupling capacitor C0 so as to prevent DC (Direct Current) current from passing therethrough.
The amplifiers 13L1 and 13L2 are designed to amplify a signal fed to a gate G thereof and to output the signal from a drain D when, for example, bias voltages xe2x88x92B1 and +B1 are applied to the gate G and the drain D via bias lines 16L1 and 16L2 respectively. In this case, a source of the GaAsFET is connected to ground (not illustrated).
Coupling capacitors C1 to C5 are formed in the microstrip line 14R to separate the bias lines 16L1, 16L2, 16R1, and 16R2 from each other as an independent DC line. The amplifiers 13R1 and 13R2 are designed to amplify a signal fed to the gate G thereof and to output the signal from the drain D when, for example, bias voltages xe2x88x92B2 and +B2 are applied to the gate G and the drain D respectively. In this case, the source of the GaAsFET is connected to ground (not illustrated).
However, according to the aforementioned low-noise high-frequency amplifier 10, the microstrip lines 14L and 14R formed on the substrate are made thin and, in addition, a width W thereof is also made small so as to increase overall packaging density. As a result, facing electrodes of each capacitor should be made longer in the longitudinal direction of the microstrip line 14R so that each of the capacitors C1 to C5 gains a sufficient facing area for having a specified capacitance.
The longer microstrip line 14R becomes, the more apart the bias lines 16L1 and 16L2 are spaced out from each other. Consequently, the overall length of the microstrip line 14L becomes longer, resulting in an unduly larger low-noise high-frequency amplifier 10 in size. A similar drawback is also seen even in the case where the capacitors C1 to C5 can be formed without elongating the facing electrodes thereof in the longitudinal direction of the microstrip line 14R, because the gaps lying between the facing electrodes of the capacitors are added up to an existing length thereof.
Although it is not illustrated, in order to prevent the bias lines from crossing one of the microstrip lines on an obverse side of the substrate, the bias lines should be routed in two directions, one from upper side and the other from lower side of the substrate (FIG. 5). Accordingly, components mounted on the substrate for supplying the bias voltage occupy at least two areas, resulting in an unduly large circuit in size. If the bias lines are routed parallel to the microstrip lines, the components for supplying the bias voltage should be mounted at either left or right side, or both (FIG. 5), resulting in an unduly long high-frequency circuit in longitudinal direction. Therefore, it is a common practice to design a high-frequency circuit having bias lines that cross a microstrip line. As a result, not only such a circuit as the aforementioned low-noise high-frequency amplifier 10 but also any high-frequency circuit having bias lines that cross a microstrip line requires capacitors that separate each bias line as an independent DC line, thereby making the high-frequency circuit still unduly large in size.
An object of the present invention is to provide a high-frequency circuit that can be made compact even in the case where bias lines cross a microstrip line in a plan view.
To achieve the above object, according to one aspect of the present invention, a high-frequency circuit comprises a substrate having an electronic component on an obverse side thereof, a microstrip line formed on the obverse side of the substrate, and one or more bias lines connected to the electronic component on the obverse side of the substrate and formed on a reverse side of the substrate so as to cross the microstrip line in a plan view, wherein the bias lines supply bias voltages to the electronic component.
In this structure, the bias line lying on the reverse side of the substrate and crossing the microstrip line in a plan view, is insulated therefrom and supplies bias voltages to the electronic component.
According to another aspect of the present invention, in a high-frequency circuit as described above, as the bias line, a plurality of bias lines are formed so as to cross, in a plan view, a single conductor formed as the microstrip line.
According to still another aspect of the present invention, in a high-frequency circuit as described above, a direction of current passing through any one of the bias lines is opposite to a direction of current passing through the adjacent bias line.
According to still another aspect of the present invention, in a high-frequency circuit as described above, a filter is connected to the bias lines.